This invention relates generally to the field of magnetic data storage devices, and more particularly, but not by way of limitation, to an improved model reference generator for a disc drive.
Disc drives are used for data storage in modern electronic products ranging from digital cameras to computer systems and networks. A typical disc drive includes a head-disc assembly (HDA) housing the mechanical portion of the drive, and a printed circuit board assembly (PCBA), attached to the head-disc assembly. The printed circuit board assembly controls operations of the head-disc assembly and provides a communication link between the head-disc assembly and a host device served by the disc drive.
Typically, the head-disc assembly has a disc with a recording surface rotated at a constant speed by a spindle motor assembly and an actuator assembly positionably controlled by a closed loop servo system. The actuator assembly supports a read/write head that writes data to and reads data from the recording surface. Disc drives using magneto resistive read/write heads typically use an inductive element, or writer, of the read/write head to write data to the information tracks and a magneto resistive element, or reader, to read data from the information tracks during drive operations.
One type of data recorded to and read from the information tracks is servo data. Servo data, including a physical track identification portion (also referred to as a servo track number or physical track number), written to the recording surface define each specific physical track of a number of physical tracks written on the recording surface.
A servo track writer is typically used in writing a predetermined number of physical tracks, also referred to as servo tracks, to each recording surface during the manufacturing process. The servo tracks are used by the closed loop servo system for controlling the position of the read/write head relative to the recording surface during disc drive operations, such a closed loop digital servo system is disclosed in U.S. Pat. No. 5,262,907 issued Nov. 16, 1993 to Duffy et al., assigned to the assignee of the present invention.
Seeking entails the movement of a selected head from an initial track to a destination track. For seeks of a sufficient length, a velocity-control approach is typically employed wherein the velocity of the head is repetitively determined and compared to a velocity profile which defines an optimum velocity trajectory for the head as it moves to the target track. The optimum velocity trajectory is typically provided by a model reference generator, such as the model reference generator disclosed in U.S. Pat. No. 6,031,684 to Gregg, assigned to the assignee of the present invention.
Model reference generators are designed to model a desired or expected response of the plant responding to a given input. For example, a model reference generator for a disc drive is designed to model an expected response of the head disc assembly to a seek command. The head disc assembly is the plant and the seek command is the given input. The seek command comes in the form of current profile applied to a voice coil motor of the head disc assembly. Those skilled in the art will recognize that the current profile is a modified one minus cosine reference current signal that includes both positive and negative current values to first accelerate and then decelerate the read/write head from the initial track to the destination track during a seek operation.
Based on a starting position of a read/write head, a double integrator of the model reference generator provides a reference velocity vector and a reference position vector representative of a velocity vector and position vector of the read/write head responding to a given current profile applied to the voice coil motor.
Generally, the use of the term optimum in describing a velocity trajectory of a model reference generator is used to denote the status of the model as a mathematical optimized model rather than to denote the model as being optimized to mimic the actual operation of the plant being model. In other words, the reference model is set up to provide an optimum model result for a theoretical plant based on an input current. For a disc drive, the theoretical plant is a mathematically optimized head disc assembly and the output from the model is an expected position and an expected velocity that theoretical read/write heads of the mathematically optimized head disc assembly would attain based on a starting position and velocity and a given level of input current.
Typically, the model reference generator is encoded within a servo microprocessor capable of performing double integration operations and is included as part of the model reference generator. As current is an approximation of acceleration, the first integral of an input current profile provides the velocity vector for the theoretical read/write heads and the second integral of the input current profile provides the position vector of the theoretical read/write heads of the mathematically optimized head disc assembly.
To execute a model reference seek, a current profile is provided to the voice coil motor and a model reference generator. During the model reference seek, the read/write head of the head disc assembly reads position information from the servo data written to the information tracks. The position information read by the read/write head during the model reference seek is used to generate an actual position vector indicative of the position of the read/write head. The actual position vector is compared to the position vector of the theoretical read/write heads provided by the model reference generator and corrections are made by the servo system to adjust the actual position of the read/write head to comply with the theoretical position of the read/write heads.
Typically, at pre-selected time intervals during the execution of the model reference seek, position observations of the read/write heads are made. Because of the disparity between the actual operation of the head disc assembly and the mathematically predicted operation of the head disc assembly position corrections for the read/write head are generally made for every position observations made. Extended seek times and elevated energy consumption result from the continual corrections made by the servo system to bring the position of the read/write heads into compliance with the mathematically determined reference position for the read/write heads.
Therefore, challenges remain and needs persist for means and steps for reducing the occurrence and degree of corrections made during a model reference seek to improve seek times and reduce energy consumption.
As exemplified by preferred embodiments, the present invention provides an improved model reference generator of a servo circuit for use in controlling the operation of a head disc assembly, also referred to herein as a plant, of a data storage device. The improved model reference generator is determined by providing an initial model reference generator, which includes an empirically determined filter coefficient contained in a table, to the servo system. Then a current profile is applied to the model reference generator and to the plant. The plant is operated in an open loop control mode while the current profile is applied to the plant and the response of the plant is observed while the plant is running in the open loop control mode. While the response of the plant running in the open loop control mode is observed, a response of the model reference generator to the application of the current profile to the model reference generator is determined. An optimized filter coefficient is determined from the response of the plant and the model reference generator to the application of the current profile. The empirically determined filter coefficient is replaced with the optimized filter coefficient to provide the improved model reference generator substantially replicating the operation of the plant for use in controlling the operation of the plant.
The servo circuit includes a microprocessor with an associated servo random access memory that stores a current profile and a set of optimized filter coefficients associates with the current profile. The optimized filter coefficients optimize a delay length of a finite impulse response filter, which delays passage of the current profile to a double integrator embedded in the microprocessor. The double integrator function of the microprocessor generates the substantially mimicked head positions.